The present invention relates generally to radio-frequency (RF) synthesizers, and more particularly to RF synthesizers for duplex radios.
In duplex radios, information signals, including voice, tone or data signals, may be simultaneously transmitted and received. Duplex operation is achieved in radios by utilizing two RF carrier signals having frequencies separated by a preselected frequency spacing. For example, in the 450-470 mHz frequency band, the transmit carrier signal has a frequency that is spaced 5 mHz from the receive carrier frequency. Conventional mobile radios exhibiting duplex operation are described in Motorola Instruction Manual Numbers 68P81020E10, 68P81029E65 and 68P81039E25. These Motorola Instruction Manuals and the others referenced hereinbelow may be obtained from Motorola Service Publications Department, Motorola, Inc., 1301 East Algonquin Road, Schaumburg, Ill.
In conventional duplex radios, a frequency synthesizer has been used to generate receive and transmit injection signals. The receive injection signal may be coupled to a mixer for combination with the receive carrier signal for developing an intermediate frequency signal, which may then be demodulated to recover information signals modulated on the receive carrier signal. The transmit injection signal is typically coupled to a mixer for combination with an offset signal provided by a separate oscillator for generating the transmit carrier signal. The transmit carrier signal can be modulated with information signals by modulating the offset oscillator. Radios including this type of offset oscillator are described in U.S. Pat. No. 3,825,830 and in the aforementioned Motorola instruction manual No. 68P81029E65.
Since the transmit carrier signal is developed by mixing the transmit injection signal with the offset signal, the frequency stability of the transmit carrier signal is affected by both the frequency stability of the transmit injection signal and the frequency stability of the offset oscillator. In order to meet Federal Communications Commission regulations on the frequency stability for the transmit carrier signal, relatively high stability oscillators must be used for both the reference oscillator of the frequency synthesizer and the offset oscillator. Such high stability oscillators are relatively expensive. Furthermore, the cost of these high frequency stability oscillators increases as their frequency of operation increases. Moreover, the frequency stability regulations of the Federal Communications Commission are stricter as the frequency of operation is increased. Thus, relatively expensive oscillators are required for radios having carrier frequencies in the 400 mHz frequency range and in the new 800 mHz frequency range.